This invention relates to a class of GNSS applications such as, but not limited to, the following:                1. Syntonization (i.e., locking to a precise frequency reference) and synchronization (i.e., locking to a precise timing reference) of short range wireless communication access points or base stations (such as wireless communications picocells and femtocells),        2. Location of shipping containers,        3. Location of cryogenic transport containers, and        4. Location, syntonization, and synchronization of seismic sensors used in arrays for geological surveying.        
Such applications are characterized as follows:                1. The GNSS receiver is embedded in a piece of portable or transportable equipment which may not be moving or moving slowly when in use;        2. The GNSS antenna is co-located with or nearby the receiver equipment;        3. The sky is generally at least partly obscured at the GNSS antenna;        4. The GNSS receiver has access to the internet or to a Wireless LAN, from which it can receive assistance;        5. The GNSS receiver provides syntonization, synchronization, location, or a combination to the host equipment.        
These characteristics differ from those of other Assisted GPS (AGPS) applications in a number of respects. Most AGPS applications relate to the location of mobile equipment and, especially, to the location of cellular handsets. Such devices are often in motion when in use. Because the receivers communicate with a base station, their approximate location can be determined by the location of the base station, whereas in the class of application addressed by the present invention, there often is no such source of location assistance. Also, the present invention is capable of providing precise synchronization and syntonization whereas most AGPS applications only provide location.
This class of applications will be referred to from here on as the “Target Applications.”
GPS signals incorporate a spreading code that is multiplied into the signal and repeated every millisecond. All GPS and AGPS receivers operate by generating a local replica of the spreading code for each satellite signal to be tracked and they align the local replica code with the incoming spreading code via a correlation technique. Once aligned, the spreading code is removed from the incoming signal by simply multiplying the signal by the replica code. In a conventional GPS receiver, the phases of the replica codes (which are the same as those of the incoming signals) are all latched at each measurement instant along with the Time Of Receipt (TOR) given by the receiver's clock.
In a conventional receiver the code epochs in each data bit are counted, the bits are counted in the words, and the words are counted in the subframes. Hence it is possible to determine exactly how many code epochs have elapsed since the last subframe boundary. The elapsed number of code epochs can be combined with the codephase at the measurement instant to determine the exact elapsed time (in satellite time) between the last subframe boundary and the measurement instant. This term is added to the time tag of the subframe boundary to give the exact TOT corresponding to the measurement instant. This term is subtracted from the latched TOR, with the result multiplied by the speed of light to yield the pseudorange.
The pseudoranges from multiple satellites are used along with the satellite locations at the corresponding TOTs to estimate the location of the receiver and the error in the TOR. This determination allows the receiver clock to be very precisely calibrated so that it can be used for both synchronization and syntonization.
AGPS location solutions differ from traditional GPS location solutions due to the use of ambiguous codephases instead of full TOTs for each GPS satellite observation. Because the signals are often too weak to allow the data to be adequately or completely extracted, it is not possible to accurately count the code epochs from the bit boundaries, to count the bits, or to count the words from the subframe boundaries. Therefore it is necessary to reconstruct the full TOT using some other means.
In the prior art, the full TOT is reconstructed using apriori knowledge, such as a position estimate within 75 km of the true position and an estimate of TOR. Errors arise if the initial TOR used to construct the TOT observations are in error. Gross errors can also result from incorrect codephase ambiguity resolution if the initial position error is larger than 75 km. The present invention resolves both of these difficulties.
AGPS is able to perform a navigation solution through the use of prior information, including a rough estimate of the position of the receiver and a time tag for the TOR. When the receiver is in communication with a base station, the rough estimate of position of the receiver is generally based on the location of the cell site. These parameters can then be used to estimate ranges to each satellite which together with the initial TOR estimate can be used to estimate a full TOT for each satellite. However, since the initial TOR typically contains errors, the reconstructed TOTs will all be subject to a common timing error thereby resulting in navigation position errors in the solution process.
The problem of time-recovery for AGPS is discussed in J. Syrjarinne, “Possibilities for GPS Time Recovery with GSM Network Assistance,” presented at ION GPS 2000, Salt-Lake City, Utah, 2000, and M. M. Chanarkar, “Resolving time ambiguity in GPS using over-determined navigation solution.” United States of America: Sirf Technology, Inc, 2003. This reference outlines general algorithms for solution of the timing error through addition of an additional variable and its solution using least squares techniques.
Approaches, such as by Surjarinne, are adequate to reduce the common timing error to the point that the corresponding position error is small. The latter results from the satellite positions being incorrectly calculated because of the common timing error. If the common timing error is only a few milliseconds, then the resulting receiver location will only be incorrect by a few meters. However, if the receiver is to be used for synchronization, timing errors of a few milliseconds are unacceptable. The millisecond ambiguity must be completely and reliably resolved. The present invention provides this level of reliability in the ambiguity resolution.
Another approach is disclosed in U.S. Pat. No. 6,459,407. However, in that scheme, as in much of the prior art, an approximate location accurate enough to assist in resolving the codephase ambiguity is supplied from a cellular base station which is not the case in many of the target application of the present invention. In contrast to U.S. Pat. No. 6,459,407, the present invention does not presume the availability of a GPS-enabled base station used for determining the location of a cellular terminal, having access to location assistance that is accurate enough to assist in resolving the codephase ambiguity.
An approach to the complete resolution of the codephase ambiguity within the prior art relies on at least one satellite signal being strong enough to permit full resolution of the TOT in the conventional way. If the position is also known to within 75 km then the TOT differences between any two satellite signals at the measurement instant cannot be more than 150 km divided by the speed of light or 0.5 milliseconds. Thus it is relatively easy to establish the TOT of all other satellite signals based on that of the first one and the codephases of the other signals at the measurement instant.
That approach, however, suffers from serious drawbacks for use in the applications for which the present invention is intended. First, the GPS receiver antenna may often be located where it is not possible to receive one signal strong enough to permit data extraction and resolution of the TOT in the conventional way. Second, the position of the receiver antenna may not be known to an accuracy of 75 km. Indeed, it may not be known to better than a few thousand km.